Electronic imaging devices commonly include a plane semi-conductive image sensor made of silicon using CMOS or CCD technology, and an optic which forms an image of the scene observed on the image sensor.
However, the use of a straightforward converging lens as an optic is not satisfactory in so far as the image formed by said lens is not plane but spherical, a phenomenon known as “curvature of field”. In fact, the image projected by a converging lens on a plane sensor is either sharp at the center but not at the edges or vice versa. This explains in particular why complex optics are produced, consisting of lens clusters that have additionally been subjected to specific surface treatments so that the images they produce conform to the plane nature of the sensor.
However, even at the present time, the most complex optics still introduce a certain number of both geometric and chromatic aberrations, among which can be numbered barrel and pincushion distortions, spherical aberrations (or so-called “diffused light” aberrations), coma, astigmatism, vignetting, glare, stray light (reflection), or chromatic fringes.
Such aberrations need to be corrected, directly upon image formation, through the use of complex and bulky optics, and/or after the event, through the implementation of image processing algorithms that require considerable processing power. The plane nature of the sensors is thus the direct cause of aberrations and the correction thereof involves bulky and expensive lenses and powerful processors built into the cameras and digital photographic appliances.
An effective way of eliminating the errors resulting from the curvature of field is to modify the shape of the image sensor so that it is substantially the same shape as the image formed by the optic. The fact that the sensor is able to be curved therefore means that not only can the aberrations be corrected, but also that cameras and photographic appliances can be designed that are compact and cheap and do not require huge amounts of processing power, accompanied overall by increased visual sharpness of up to 180° for lenses of the “fish-eye” type.
The advantage of designing curved sensors in the image formation field is thus easily conceivable.
Commonly, digital sensors, whatever the technology (CCD or CMOS for visible light, CdHgTe-based for infrared, etc.), and configuration (monolithic, hybridized, etc.) thereof, include a substrate in which a pixel read-out circuit is formed, said substrate having a thickness of between few tens of micrometers and a plurality of millimeters.
In fact, it remains difficult to produce a curved substrate, or more generally to produce a flexible circuit, in respect of thicknesses such as these.
Indeed, curving a plane circuit of significant thickness (typically above 50 micrometers), and therefore of significant rigidity, causes defects that are detrimental to the quality of the circuit, such as for example piping, cracking, tearing, or even the destruction of connections and electrical components contained in the circuit.
To avoid such drawbacks, it is possible to design a circuit that has very low thickness (typically below 50 micrometers for a silicon circuit), and consequently great flexibility, and then to bond it onto a more rigid carrier that has the desired curvature.
However, the adhesive often has uncontrollable and unforeseeable defects (bubbles, defects of homogeneity, etc.) which are transferred to the thinned-down circuit on account of the low thickness thereof. The final quality of the circuit is thus largely random.
Additionally, bonding onto a curved surface is difficult since it is desirable not to put too much pressure on the circuit at the risk of damaging it, and all the more difficult since said surface has a large number of variations. Commonly therefore, bonding is restricted to a plane or slightly convex surface.